Rare-earth (RE) doped optical fibers have found promising applications in the field of optical amplifiers, fiber lasers and sensors. The RE elements doped into the core of such fibers act as the active medium. Different REs like Er, Nd, Yb, Sm, Ho and Tm can be doped to get lasing and amplification covering a wide range of wavelengths. Er doped fiber amplifier (EDFA) due to its high quantum efficiency and broad gain bandwidth shows tremendous application in communication field meeting up the huge bandwidth requirement in internet services and information technology. RE-doped fiber lasers are replacing gas based or solid state lasers in most of the applications due to their compactness, excellent beam quality and easy handling capability. As a result, there has been a tremendous growth in the market with the overall sales predicted to touch $2.8 billion industrial laser market by 2010. Fiber laser devices are suitable for a variety of applications viz. material processing (cutting, grinding and engraving), range finding, medical and military applications. Thus fabrication of RE doped fibers with varied designs, compositions and appropriate RE concentration attracts a lot of research interest. The improvement in the properties of the fibers and increase in the process reproducibility remain the prime objective.
Reference may be made to Townsend J. E., Poole S. B., and Payne D. N., Electronics Letters, Vol. 23 (1987) p-329, “Solution-doping technique for fabrication of rare-earth-doped optical fiber” wherein, the Microwave Chemical Vapour Deposition (MCVD) process is used to fabricate the preform with a step index profile with desired core-clad structure, while solution doping is adopted for incorporation of the active ion. In the first step P2O5 and F doped cladding layer with desired thickness is deposited within a high silica glass substrate tube to produce matched clad or depressed clad type structure followed by deposition of core layers of predetermined composition containing index-raising dopant like GeO2 at a lower temperature to form unsintered porous soot. Deposited soot layer is then immersed into an aqueous solution of the dopant precursor (typical concentration 0.1 M) up to 1 hour. Any soluble form of the dopant ion is suitable for preparation of the solution, although rare earth halides have been mostly used. After dipping, the tube is rinsed with acetone and remounted on lathe. The core layer containing the RE is then dehydrated and sintered to produce a clear glassy layer. Dehydration is carried out at a temperature of 600° C. by using chlorine. The level of OH− is reduced below 1 ppm using Cl2/O2 ratio of 5:2 provided the drying time exceeds 30 min. Finally the tube is collapsed in the usual manner to get a solid glass rod called preform from which fiber is drawn using conventional method.
Reference may be made to DiGiovanni D. J., SPIE Vol. 1373 (1990) p-2 “Fabrication of rare-earth doped optical fiber” wherein, the substrate tube with the porous core layer is soaked in an aqueous or alcoholic solution containing a nitrate or chloride of the desired RE ion along with co-dopant Al salts. The tube is drained, dried and remounted on lathe. The dehydration is carried out by flowing dry chlorine through the tube at about 900° C. for an hour. After dehydration, the layer is sintered and the tube is collapsed to be drawn to fiber.
Reference may be made to Ainslie B. J., Craig S. P., and Wakefield B., Material Letters, Vol. 6, (1988) p-139, “The fabrication, assessment and optical properties of high-concentration Nd3+ and Er3+ doped silica based fibers” wherein to increase the rare earth solubility Al2O3—P2O5—SiO2 host glass was selected and high concentration of Nd3+ and Er3+ have been introduced using solution doping method and quantified. Following the deposition of cladding layers P2O5 doped silica soot is deposited at lower temperature. The prepared tubes are soaked in an alcoholic solution of 1 M Al(NO3)3 having various concentration of ErCl3 and NdCl3 for 1 hour. Addition of Al helps to enhance RE concentrations in the core center without clustering effect. It is further disclosed that Al and RE profile lock together in some way, which retards the volatility of RE ion.
Reference may be made to U.S. Pat. No. 7,116,472 (2006), by M. J. Andrejco and B. wang, “Rare-earth-doped optical fiber having core co-doped with fluorine” wherein a silica core region is doped with Al and fluorine (F) along with at least one rare earth element in presence of germanium. The presence of small amounts of F are effective to lower the refractive index, and hence the NA, of the core region even in the presence of significant amounts of Al (e.g., >8 mol %). This provides a fiber with a relatively flat gain spectrum and a low NA (e.g., preferably <0.20).
Reference may be made to U.S. Pat. No. 5,474,588 (1995) by D. Tanaka, A. Wada, T. Sakai, T. Nozawa and R. Yamauchi, ‘Solution doping of a silica with erbium, aluminium and phosphorus to form an optical fiber’ wherein a manufacturing method for Er doped silica is described in which silica glass soot is deposited using VAD apparatus to form a porous soot preform, dipping the said preform into an ethanol solution containing an erbium compound, an aluminium compound and a phosphoric ester, and desiccating said preform to form Er, Al and P containing soot preform. The segregation of AlCl3 in the preform formation process is suppressed due to the presence of phosphorus and as a result the doping concentration of Al ions can be set to a high level (>3 wt %). It has been also claimed that the dopants concentration and component ratio of Er, Al and P ions having extremely accurate and homogeneous in the radial as well as in longitudinal directions.
Reference may be made to U.S. Pat. No. 5,284,500 (1994) by K. Okamura and T. Arima, “Process for fabricating an optical fiber preform” wherein two constricted portions are formed at a quartz reaction tube and solution of a compound of a rare earth element is charged into the section between the constricted portions for doping which results in an uniform doping along the length of an optical fiber preform with defects being rarely produced. In order to realize a high gain over a wide wavelength band in the optical fiber amplifier using the doped fiber, it is effective to dope aluminium in the core aside from rare earth elements. Solution of an aluminum compound such as anhydrous AlCl3 is mixed with ErCl3 as a rare earth element in alcohols. Anhydrous AlCl3 is preferred in order to omit the dehydration step. The solution can also be used in the form of a mist in a soot-like core glass by which it becomes possible to control the doping concentration in high accuracy. The distribution of the doping concentration along the radial direction of the core can be arbitrarily set by controlling the deposition temperature of soot like core glass.
Reference may be made to U.S. Pat. No. 5,526,459 (1996), by D. Tanaka, A. Wada, T. Sakai, T. Nozawa and R. Yamauchi, “Erbium-doped silica optical fiber preform” wherein a silica glass soot is first deposited on a seed rod to obtain a soot preform in a porous state on the seed rod containing germanium and phosphorous oxide followed by dipping in a solution containing an erbium compound, and an aluminum compound (AlCl3). Subsequent processing provides the ultimate preform. Process helps to attain good dopant concentration and component ratio distribution of erbium ions, aluminum ions, and phosphorus ions which is extremely accurate and homogeneous in the radial and longitudinal directions, overcoming the problems existed in erbium doped optical fibers obtained via conventional methods, that is to say, the low concentration of erbium ions and aluminum ions in the core region, are solved.
Reference may be made to U.S. Pat. No. 6,751,990 (2004), by T. Bandyopadhyay, R. Sen, S. K. Bhadra, K. Dasgupta and M. Ch. Paul, “Process for making rare earth doped optical fiber” wherein unsintered particulate layer consist of GeO2 and P2O5 and soaked into an alcoholic/aqueous solution of RE-salts containing co-dopants like AlCl3/Al(NO3)3 in definite proportion. Porosity of the soot, dipping period, strength of the soaking solution and the proportion of the codopants are controlled to achieve the desired RE ion concentration in the core and to minimize the core clad boundary defects. The RE ion distribution in the resulting fibers matches with the Gaussian distribution of the pump beam to increase the overlapping and pump conversion efficiency.
Reference may be made to U.S. Pat. No. 6,851,281 (2005), by R. Sen, M. Pal, M. C. Paul, S. K. Bhadra, S. Chatterjee and K. Dasgupta. “Method of fabricating rare earth doped optical fibre” wherein MCVD process coupled with solution doping technique is used to deposit porous silica soot layer containing GeO2, P2O5 or such refractive index modifiers by the backward deposition method for formation of the core. The deposited particulate layer is presintered by backward pass with flow of GeCl4 and/or corresponding dopant halides and is soaked into an alcoholic/aqueous solution of RE-salts containing codopants such as AlCl3 in definite proportion followed by conventional steps to obtain the final preform. The fiber was drawn from preform in a usual method maintaining suitable core-clad dimensions and geometry.
The drawbacks of the above mentioned processes are as follows:
1. Doping of Al for increase in the RE solubility is associated with segregation of Al2O3 rich phase if Al ion concentration exceeds a minimum level, usually 1-3 wt % depending on fabrication conditions.
2. Increasing RE incorporation by addition of more Al in the soaking solution leads to imperfections at the core-clad boundary.
3. The problems stated under 1 and 2 are more prominent in case of Al doped germano-silicate fibers.
4. Codoping of Phosphorous reduces segregation of Al but enhances base loss of the resulting fibers.
5. The problems discussed under 1 to 3 cause degradation in optical properties of the fibers.